Well he's 7, so I don't think he needs a mobile internet device or a mobile pron-WMV player, which is why I didn't suggest getting the psp...
But since he has a DS, what's the problem with it? Get him some more games. PSP is kind of overrated. If he'll get bored with a DS he'll probably get bored with a PSP too.
NEVER buy a handheld off ebay. You have no idea how something like that's been abused. Dropped, scratched, submerged under (toilet) water, etc...A console is a noticable risk but a handheld i'd never buy used...